An electric arc furnace comprises a relatively short, wide, cylindrical enclosure having a relatively wide and shallow hearth within which metal, such as steel, is melted. Generally the furnace bottom is lined with two layers of refractory comprising a lower layer of refractory, which is intended to last for a long time, such as several years, and an upper layer of refractory, which comes in contact with molten metal and which is intended to last for only a small number of heats before replacement or patching. An electric arc furnace also has at least one electrode, and generally has three electrodes, which provide an electric arc among themselves and/or between themselves and localized areas of the metal which is to be melted within the hearth. These arcs heat and melt the metal. After the metal is melted it is poured out of the melting furnace and into a ladle from which it is poured into molds to produce cast products or into a refining vessel to produce further refined metal.
Since the arc heating within the furnace hearth is localized, the sections of the hearth furthest from the electrodes receive heat at a slower rate than areas closer to the electrodes, and the metal at such distant sections is slower to melt resulting in longer melt times with consequent high energy consumption. Furthermore, the localized nature of the arc heating and melting causes chemical and thermal stratification through the depth of the molten metal, which may cause operational problems due to errors in characterizing the molten bath because of its inhomogeneous nature.
Those skilled in the art of melting metal in an electric arc furnace have addressed this problem of inefficient melting in a number of ways.
One such method comprises the use of plasma or oxy-fuel burners or oxygen lances to produce additional heat directed at the cold regions within the furnace hearth. While such auxiliary burners may be effective, they are also complicated and costly to operate and their effectiveness decreases when larger size pieces of metal are to be melted. Furthermore, the burners or lances may oxidize the metal within the hearth which decreases the yield of the melting process and further decreases the process efficiency.
Another method employed by steelmakers to increase melting efficiency in an electric arc furnace is to stir the molten metal so that it flows over and around the not yet melted metal thus increasing the heat distribution rate and consequently increasing the melting rate.
One known method of stirring the hearth contents of an electric arc furnace is by induction stirring wherein a magnetic field is set up within the hearth and the metal is moved or stirred magnetically. Disadvantages of this method include the expense of the required induction stirring coils and controls, the necessity of providing the furnace with a non-magnetic, such as a stainless steel, bottom, the susceptibility of the induction stirring device to damage caused by inadvertent leaks or overflow of molten metal, and the fact that induction stirring can provide only a certain degree of stirring which may be less than the degree of stirring desired.
Another known method of stirring the hearth contents of an electric arc furnace is by injection of inert gas into the molten metal through a permeable or porous element such as a refractory piece. Disadvantages of this method include unreliability and the likelihood of the permeable or porous element becoming blocked, complicated and costly installation requirements, the possibility of sudden failure leading to steel leakage out the bottom of the furnace, and incompatibility with the use of carbon dioxide as the stirring gas due to a tendency of carbon dioxide to oxidize and erode the porous refractory element.
It is therefore an object of this invention to provide a method to melt metal in an electric arc furnace with improved efficiency over heretofore available melting methods.
It is another object of this invention to provide an improved melting furnace to enable melting of metal in an electric arc furnace with improved efficiency.